Proprotein convertase subtilisin kexin 9 (PCSK9) is a member of the subtilisin serine protease family. The other eight mammalian subtilisin proteases, PCSK1-PCSK8 (also called PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, and S1P/SKI-1) are proprotein converges that process a wide variety of proteins in the secretory pathway and play roles in diverse biological processes (Bergeron, F. (2000) J. Mol. Endocrinol. 24, 1-22, Gensberg, K., (1998) Semin. Cell Dev. Biol. 9, 11-17, Seidah, N. G. (1999) Brain Res. 848, 45-62, Taylor, N. A., (2003) FASEB J. 17, 1215-1227, and Zhou, A., (1999) J. Biol. Chem. 274, 20745-20748). PCSK9 has been proposed to play a role in cholesterol metabolism. PCSK9 mRNA expression is down-regulated by dietary cholesterol feeding in mice (Maxwell, K, N., (2003) J. Lipid Res. 44, 2109-2119), up-regulated by statins in HepG2 cells (Dubuc, G., (2004) Arterioscler. Thromb. Vase. Biol. 24, 1454-1459), and up-regulated in sterol regulatory element binding protein (SREBF) transgenic mice (Horton, J, D., (2003) Proc. Natl. Acad. Sci. USA 100, 12027-12032), similar to the cholesterol biosynthetic enzymes and the low-density lipoprotein receptor (LDLR). Furthermore, PCSK9 missense mutations have been found to be associated with a form of autosomal dominant hypercholesterolemia (Hchola3) (Abifadel, M., et at (2003) Nat. Genet, 34, 154-156, Timms, K. M., (2004) Hum. Genet 114, 349-353, Leren, T. P. (2004) Clin. Genet. 65, 419-422), PCSK9 may also play a role in determining LDL cholesterol levels in the general population, because single-nucleotide polymorphisms (SNPs) have been associated with cholesterol levels in a Japanese population (Shioji, K., (2004) J. Hum. Genet. 49, 109-114).
Autosomal dominant hypercholesterolemias (ADHs) are monogenic diseases in which patients exhibit elevated total and LDL cholesterol levels, tendon xanthomas, and premature atherosclerosis (Rader, D. J., (2003) J. Clin. Invest. 111, 1795-1803). The pathogenesis of ADHs and a recessive form, autosomal recessive hypercholesterolemia (ARH) (Cohen, J. C., (2003) Curr. Opin. Lipidol. 14, 121-127), is due to defects in LDL uptake by the liver, ARH may be caused by LDLR mutations, which prevent LDL uptake, or by mutations in the protein on LDL, apolipoprotein B, which binds to the LDLR. ARH is caused by mutations in the ARM protein that are necessary for endocytosis of the LDLR-LDL complex via its interaction with clathrin. Therefore, if PCSK9 mutations are causative in Hchola3 families, it seems likely that PCSK9 plays a role in receptor-mediated LDL uptake.
Overexpression studies point to a role for PCSK9 in controlling LDLR levels and, hence, LDL uptake by the liver (Maxwell K. N. (2004) Proc. Natl. Acad. Sci. USA 101, 7100-7105, Benjannet, S., et al. (2004) J. Biol. Chem. 279, 48865-18875, Park, S. W., (2004) J. Biol. Chem. 279, 50630-50638). Adenoviral-mediated overexpression of mouse or human PCSK9 for 3 or 4 days in mice results in elevated total and LDL cholesterol levels; this effect is not seen in LDLR knockout animals (Maxwell K. N. (2004) Proc. Natl. Acad. Sci. USA 101, 7100-7105, Benjannet, S., et al. (2904) J. Biol. Chem. 279, 48865-48875, Park, S. W., (2004) J. Biol. Chem. 279, 50630-50638). In addition, PCSK9 overexpression results in a severe reduction in hepatic LDLR protein, without affecting IDLE mRNA levels, SREBP protein levels, or SREBP protein nuclear to cytoplasmic ratio. These results indicate that PCSK9, either directly or indirectly, reduces LDLR protein levels by a post transcriptional mechanism
Loss of function mutations in PCSK9 have been designed in mouse models (Rashid et al., (2005) PNAS, 102, 5374-5379, and identified in human individuals Cohen et al., (2005), Nature Genetics., 37.161-165. In both cases loss of PCSK9 function lead to lowering of total and LDLc cholesterol. In a retrospective outcome study over 15 years, loss of one copy of PCSK9 was shown to shift LDLc lower and to lead to an increased risk-benefit protection from developing cardiovascular heart disease (Cohen et al. 2006 N. Engl. J. Med., 354, 1264-1272.). Clearly the evidence to date indicates that lowering of PCSK9 levels will lower LDLc.
Despite significant advances in the field of RNAi and advances in the treatment of pathological processes which can be mediated by down regulating PCSK9 gene expression, there remains a need for agents that can inhibit PCSK9 gene expression and that can treat diseases associated with PCSK9 gene expression.